RISC-V: linux-user support for RVE ABI

This change checks elf_flags for EF_RISCV_RVE and if
present uses the RVE linux syscall ABI which uses t0
for the syscall number instead of a7.

Warn and exit if a non-RVE ABI binary is run on a
cpu with the RVE extension as it is incompatible.

Backports relevant parts of 5836c3eccedb6dfab16b8f606f2de24b8938b69c
from qemu

qemu/target/riscv/cpu.h

@@ -125,6 +125,10 @@ struct CPURISCVState {
 
     uint32_t features;
 
+#ifdef CONFIG_USER_ONLY
+    uint32_t elf_flags;
+#endif
+
 #ifndef CONFIG_USER_ONLY
     target_ulong priv;
     target_ulong resetvec;

qemu/target/riscv/cpu_user.h

@@ -10,4 +10,5 @@
 #define xA4 14
 #define xA5 15
 #define xA6 16
-#define xA7 17  /* syscall number goes here */
+#define xA7 17  /* syscall number for RVI ABI */
+#define xT0 5   /* syscall number for RVE ABI */